1. Technical Field
The present disclosure relates generally to isotope measurement, and more specifically to techniques for using spectrometry to measure an apparent equilibrium constant involving a clumped isotope.
2. Background Information
Isotopologues are species of a molecule that differ in the isotopic identity of one or more of their constituent atoms. For example, 14N14N, 15N14N and 15N15N are three isotopologues of a nitrogen diatomic molecule. Multiply-substituted isotopologues, commonly referred to simply as “clumped isotopes”, are isotopologues that contains two or more rare isotopes. For example, 12C16O17O, 13C16O17O, and 13C16O18O are examples of clumped isotopes of carbon dioxide. By analyzing the relative abundances of these isotopes, or more specifically equilibrium constants involving clumped isotopes governing the formation of a sample, various types of useful information may be determined. Among other things, they may provide the formation temperature of the sample.
Traditionally, measurements of the relative abundances of clumped isotopes have been performed using mass spectrometers. More recently, attempts have been made to utilize instruments (e.g., isotope monitors) that include laser absorption spectrometers. FIG. 1 is a generalized block diagram of an example instrument 100. The example instrument includes a laser 110, a sample cell 120 having valved gas inlet and gas outlet ports 122, 124, and a light detector 130. A gaseous sample to be analyzed is fed into the sample cell by opening a valve of the gas inlet port 122. A laser beam 112 is then emitted from the laser 110 and enters the sample cell 120 through an entrance window 126. The laser beam 112 interacts with the gaseous sample, and may be partially absorbed by the gas. A remaining portion of the laser beam 112 emerges from an exit window 124, where it is detected by the light detector 130. The light detector converts the detected laser light to an electrical voltage.
The isotope monitor utilizes a computing system 140 that monitors the electrical voltage returned from the light detector. The computing system 140 may communicate with the laser 110 and direct it to change the wavelength of the laser beam within a given range to probe various spectroscopic lines being studied.
FIG. 2 is a plot showing species of a carbon dioxide sample measured using the example instrument of FIG. 1. The species include 12C16O16O (abbreviated “626”), 13C16O16O (abbreviated “636”), 12C16O18O (abbreviated “628”), 12C16O17O (abbreviated “627”), the clumped isotope 13C16O18O (abbreviated “638”), the clumped isotope 13C16O17O (abbreviated “637”), and the clumped isotope 12C8O18O (abbreviated “828”). While all required species are present in FIG. 2, the dynamic range is larger than desired. For example, the spectroscopic line strengths of the major species 636 and the clumped isotope 638 have a ratio of 40:1. This may present various challenges. For example, if the major specie's absorbance is constrained to less than 1 during testing to avoid saturation of optical absorbance, then 638 absorption will be constrained to be less than 2.5%. A spectroscopic line with 2.5% absorbance must be measured at extremely low noise levels (which are difficult to achieve with conventional instruments) to yield desirable levels of sensitivity. Greater sensitivity may be possible if absorbance could be constrained to a range of 0.1 to 1.0 for all required species. However, achieving more uniform absorbance among all required species presents a significant challenge.
Accordingly, there is a need for improved techniques for using laser absorption spectrometry to measure relative abundances of clumped isotopes, or more specifically equilibrium constants involving clumped isotopes.